1. Field of the Invention
The present invention relates to a viewfinder optical system which can be advantageously used for an SLR camera (e.g., an SLR camera using silver-halide film or a digital SLR camera) in which the distance between the image plane (e.g., the sensitive surface of an image pick-up device or a silver-halide film) and the back face of the camera has to be relatively long.
2. Description of the Related Art
SLR cameras are generally provided with a viewfinder optical system through which the object image formed on a focusing glass (i.e., a focusing screen) positioned above the mirror box is viewed as an erect image at the eye of the user (photographer). In most cases, the viewfinder optical system is of a type which includes a pentagonal roof prism and an eyepiece positioned behind the exit surface of the pentagonal roof prism. In this type of viewfinder optical system, an optical axis of the viewfinder optical system which extends through, and perpendicularly to, the approximate center of the focusing glass, is bent rearward via the pentagonal roof prism so as to extend parallel to the optical axis of the photographic lens to enter the eyepiece. The object image formed on the focusing glass is magnified by the eyepiece to be viewed by the user.
Due to the recent popularization of digital cameras, there has been an attempt to combine the advantage of an image processing function, which is unique to digital cameras, with the advantage of the optical system being free from parallax error, which is unique to SLR cameras, by combining the structure of a digital camera with the structure of an SLR camera. For example, there has been an attempt to replace the back lid of a conventional SLR camera with a back lid which has a CCD image sensor, an electric circuit and a data recording medium (e.g., a flash memory card or a floppy disk drive) provided therein, so that the conventional SLR camera can be used as an SLR digital camera. Furthermore, there has been an attempt to design the body of an SLR camera to be used exclusively as a digital camera body in which a CCD image sensor is fixed at a position at which sensitive film was formerly positioned in a conventional SLR camera.
FIG. 2 is a schematic illustration showing the arrangement of fundamental elements of a typical SLR digital camera. As shown in FIG. 2, a typical SLR digital camera is provided, behind a mirror box 12, with a focal-plane shutter unit 13, a CCD image sensor 14 and a recording medium 15 which are arranged in this order from the side of the mirror box 12, which accommodates a quick-return mirror 11 therein. The SLR digital camera is provided above the mirror box 12 with a focusing glass 16 and a pentagonal roof prism 17 which are arranged in this order from the side of the mirror box 12. The SLR digital camera is further provided behind the exit surface 17a of the pentagonal roof prism 17 with an eyepiece 18. In the SLR digital camera having such a structure, the distance between the image plane (i.e., the sensitive surface of the CCD image sensor 14), on which the object image is formed through the photographic lens 19, and a back face C of the SLR camera is much longer than that of a typical SLR camera using silver-halide film.
The distance between the image plane and the back face of a multi-functional SLR camera using silver-halide film produced in recent years is also longer than that of a conventional simple SLR camera because such a multi-functional SLR camera often have various electric circuits, mechanisms and/or specially-prepared accessories provided behind the film plane.
In an SLR digital camera in which the distance between the sensitive surface of the CCD image sensor and the back face of the camera is long, a problem arises with the eyepiece of the camera. This problem will be hereinafter discussed.
In FIG. 2, in the case where the eyepiece 18 is arranged behind the pentagonal roof prism 17 at a position "A" shown by a dotted line, namely, wherein the eyepiece 18 is arranged at a position substantially identical to the position at which the eyepiece of a conventional SLR camera using silver-halide film is arranged, part of the camera body in which the CCD image sensor 14 and the recording medium 15 are accommodated bulges rearward by a considerable amount. Accordingly, when the user aligns his/her eye with the eyepiece, the camera body tends to interfere, especially around the cheek area. As a result, the user cannot look into the eyepiece 18 with his or her eye positioned closely to the eyepiece 18, which makes it difficult for the user to view through the eyepiece 18. To eliminate this problem, the viewfinder optical system can be designed so that the exit pupil of the eyepiece 18 is formed at a position on a plane, which includes the back face of the camera, by establishing a long eye relief of the eyepiece 18 (distance between the rear surface of the eyepiece 18 and the eye point thereof). However, according to such a design, both the lens diameter of the eyepiece 18 and the size of the pentagonal roof prism 17 must be increased, which makes the camera bulky (i.e., the physical size thereof is increased).
In order to eliminate the aforementioned problem without increasing either the lens diameter of the eyepiece 18 or the size of the pentagonal roof prism 17, if the eyepiece 18 is fixed at a position "B" shown by a solid line in FIG. 2, the focal length of the eyepiece 18 must be made longer than that in the case where the eyepiece 18 is fixed at the position "A" (i.e., the focal length of the eyepiece 18 must be extended by a length corresponding to the distance between the positions A and B). As a result, the magnification of finder decreases to thereby arise a problem of decreasing the apparent visual angle relative to the subject.